Anionic ring-opening polymerization of epoxides: kinetics, reactivity ratios, and renewable monomer strategies
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Abstract
Anionic ring-opening polymerization (AROP) of epoxides is a fundamental method for developing
innovative and customized materials. Established for more than a century, it is widely used in both
industry and academia. Poly(ethylene oxide) (PEO) or poly(ethylene glycol) (PEG), with its
exceptional aqueous solubility, enables the formation of amphiphilic block copolymers that
compatibilize hydrophobic molecules with water. Hydrophobic and hydrophilic comonomers can
be varied over a vast range towards the desired application. Statistical copolymerization of epoxides
allows for combining the properties of different comonomers, with ethylene oxide commonly used
due to its hydrophilic nature.
In recent years, the synthesis of bio-based compounds from renewable and abundant resources
has gained significant attention. Terpenoids provide an accessible platform for synthesizing a
variety of glycidyl ethers, offering structural diversity and modifiability for creating customized
materials. Fatty alcohols complement this by providing linear hydrophobic monomers, which can
be subsequently modified if a double bond is present. The copolymerization of epoxide monomers
requires detailed investigations into incorporation preferences to understand the resulting
structure-property relationships. This thesis presents a comprehensive review of the existing
literature on epoxide copolymerization and explores the copolymerization of various monomers
suited for developing innovative materials. Particular emphasis is placed on the synthesis and
inquiry of bio-based monomers.
Chapter 2 provides a general introduction to copolymerization, focusing on the underlying kinetic
aspects. It presents the official terminology for copolymers and reviews the commonly used but
imprecise term "random copolymer", offering a more accurate definition. Distinct mathematical
models applicable to describing copolymerization are discussed, along with recommendations for
their use. Copolymers with a gradient distribution of comonomers are highlighted as a class at the
boundary between random and block copolymers. Depending on the copolymerization method,
conventional or monomer-activated anionic ring-opening polymerization, control over the
incorporation preferences is allowed within certain limits. The centerpiece of this chapter
represents the tables of copolymerization behavior of all available epoxide comonomer
combinations gathered from the literature.
Chapter 3 investigates how polymerization conditions influence the incorporation preferences of
ethylene oxide (EO) and glycidyl methyl ether (GME). Copolymers of EO and GME, a “dimeric
isomer” of EO, are explored as a potential alternative to PEG for biomedical applications. Variables
such as solvent choice can shift the incorporation of preferences of EO and GME from a random
copolymerization to one with a slight gradient. This shift is significant for synthesizing copolymers
with specific monomer distributions, as anti-PEG antibodies bind to certain motifs of consecutive
EO units. The crystallinity of the bulk material depends on the monomer distribution, which is
crucial for applications requiring amorphous polymers, such as solid-state batteries.
Chapter 4 examines the influence of copolymerization conditions on the well-established EO and
propylene oxide (PO) comonomer pair, which has been used industrially for decades. Despite its
decades-long use, comprehensive data on melting points and aqueous solubility are unavailable.
Even small variations in incorporation preferences impact aqueous properties. PO is incorporated
slower than EO, leading to pronounced gradient formations. Copolymers with a steeper gradient
were better soluble in water compared to those with a smoother gradient.
Chapter 5 explores bio-based terpenyl glycidyl ethers, which offer a renewable alternative to
conventional petro-based monomers, contributing to the goal of decarbonizing the chemical
industry. The copolymerization of short- to medium-chain acyclic terpenyl glycidyl ethers with EO
was investigated. Despite the structural diversity of these monomers, similar incorporation
preferences with EO were observed, although the more apolar compounds exhibited slower
incorporation rates. The double bonds of the terpenyl glycidyl ethers were functionalized by thiolene
click reaction using 2-mercaptoethanol as a model compound, making these materials suitable
for introducing virtually any functional group for tailor-made applications. Reduction of the double
bonds with diimide enabled subsequent saturation of the side chains, making these materials less
prone to aging.
Chapter 6 focuses on synthesizing oleyl glycidyl ether (OlGE) from oleyl alcohol and
epichlorohydrin. Similar to terpenyl glycidyl ethers, monomers derived from fatty alcohols serve as
valuable resources for hydrophobic monomers. Their linear structure allows saturated long-chain
variants to solidify at room temperature, while medium-chain or cis-unsaturated variants remain
liquid. Despite its highly apolar and bulky side chain, OlGE exhibited only a slightly lower
incorporation preference when copolymerized with EO. Block and statistical copolymers of OlGE
and EO were investigated for their micellization behavior. By incorporating just a few mol% of this
hydrophobic comonomer with a high molar mass, a wide range of hydrophilic-lipophilic balances
could be achieved. Transmission electron microscopy (TEM) and dynamic light scattering (DLS)
revealed the formation of micelles, which assembled into larger aggregates. The double bond in
OlGE was accessible to thiol-ene click reactions and reduction via diimide, allowing partial or
complete reduction of the side chains. This modification enabled the fine-tuning of melting points
to fall within the physiological range, offering customizable material properties for biomedical
applications.